Interpolymers prepared by copolymerizing ethylene with a minor amount of at least one alkene in the range of C.sub.3 -C.sub.12, especially in the range of C.sub.5 -C.sub.9, using a metal or coordination catalyst, such as those of the Ziegler-type, the Natta-type, or the Phillips-type, have become known as "linear low density polyethylene" (LLDPE); the "polyethylene" portion of the expression is actually an "interpolymer of ethylene", which is a particular form of a "copolymer of ethylene". The LLDPE copolymers in this disclosure are in contradistinction to the branched-chain low density polyethylene (LDPE) polymers and the linear high density polyethylene (HDPE) homopolymers which are well known in the art.
Various patents disclose catalysts and processes which produce LLDPE polymers with various amounts of ethylene/1-alkenes having various properties. Also disclosed are various reasons for attempting to achieve certain combinations of polymer properties for various end-uses.
Considering that the present invention, which is described in detail below, pertains to the unexpected discovery that certain critical properties are required in order to achieve the desired results, then it is believed that the most relevant prior art is that which is exemplified by the following publications:
U.S. Pat. No. 4,076,698 (Anderson et al) discloses the process of making linear low density ethylene/1-alkene copolymers using metal coordination catalysts, and demonstrates that the properties are different from the branched low density ethylene homopolymers made in a high pressure reactor using a free radical catalyst. It is also disclosed that the amount and size of the 1-alkene used in making the copolymer gives densities in the "low" range as opposed to the "high" densities obtained using metal coordination catalyst in making linear ethylene homopolymers. Anderson et al disclose that a polymer with a narrow molecular weight distribution (MWD) is necessary for the best environmental stress crack resistance. See, e.g. Anderson et al col. 11, lines 65-69 which discloses that stress crack resistance is improved by synthesizing a polymer with a narrow molecular weight distribution.
J. H. Herman et al in Polymer Engineering Science page 341, October, 1966 disclosed that narrowing the MWD at constant melt index improves the stress crack resistance of polymers.
U.S. Pat. Nos. 4,192,935 and 4,294,794 disclose interpolymer compositions having a density in the range of about 0.940-0.960, a melt index in the range of 100 to 200 grams/10 minutes and a ratio of weight-average molecular weight to number-average molecular weight of less than 5 for use in injection molding thin-wall containers: this represents a narrow molecular weight distribution.
U.S. Pat. Nos. 4,230,831 (col 1, lines 7-17); 4,525,322 (col 1, lines 11-23) and 4,617,352 (col. 1, lines 13-26) teach that it is well known that polymers having low molecular weight along with a narrow molecular weight distribution are suitable for articles molded by injection molding processes while blow molding processes require polymers having relatively high molecular weights and broad molecular weight distribution. Thus it is important to distinguish which type of molding process is being encountered.
U.S. Pat. Nos. 4,593,009 and 4,672,096 by Thomas E. Nowlin disclose catalytic processes for polymerizing alpha-olefins which yield LLDPE or HDPE of relatively broad molecular weight distribution, and disclose a Melt Index range of 0.1 to 50 grams/10 minutes. (An I.sub.21 /I.sub.2 ratio in the range of about 90-240 for LLDPE is believed to correspond to an I.sub.10 /I.sub.2 ratio in the range of about 19.8 to about 46.7.)